1. Field of the Invention
The present invention relates to a degradation simulator for gas sensors.
2. Description of Related Art
There is known a degradation simulator for an oxygen sensor used for measuring oxygen concentration in an exhaust gas of an internal combustion engine, the degradation simulator having a function of setting an output of the oxygen sensor in a pseudo-degraded state for simulation purpose. According to this degradation simulator, it becomes possible to obtain an oxygen sensor (dummy sensor) in a degraded state without performing a lengthy durability test, or taking the trouble to fabricate an oxygen sensor in a degraded state.
As such a degradation simulator, Japanese Patent No. 3869338 discloses an apparatus including a signal processor and a dummy sensor, the signal processor having a function of varying, in accordance with a target degradation, an oxygen pumping current for an oxygen sensor including an oxygen concentration cell and an oxygen pumping cell (the so-called 2-cell sensor), the dummy sensor having a function of varying, in accordance with the target degradation, an element resistance signal and an output signal of the signal processor. This patent document further discloses that the signal processor includes at least one of an offset correction means for varying an offset of the oxygen pumping current, a gain correction means for varying a gain, and a response characteristic correction means for varying a delay of output change (time constant).
Meanwhile, the inventors of the present invention have found that there are various causes that degrade an output of a gas sensor other than degradation of a sensor element of the gas sensor itself. For example, when the gas sensor is exposed to an exhaust gas, adhesion of various substances to an element cover of the gas sensor degrades the output of a gas sensor. Accordingly, since the degradation states which conventional degradation simulators can simulate are limited, the conventional degradation simulators are insufficient in performance for simulating various degradation states that can actually occur in gas sensors.